Many portable digital devices incorporate at least one display screen to provide graphical information to a user or viewer. The display screen may include a liquid crystal display (LCD). Such devices may also include one or more sensors located beneath a cover glass that overlies, and typically extends beyond, the LCD. As one example, these sensors may be capacitive sensors.
The devices may also incorporate an opaque region, such as black coating region, outside the display screen (e.g., outside the active display region) but beneath the cover glass. The opaque region may include opaque ink like black ink under a cover glass or sapphire. The devices may also incorporate a button, which is one non-limiting and non-exclusive way to permit a user to provide input to the device. When the button is implemented as or incorporating a mechanical switch, it is often located within the opaque region. The same may be true when the button is a “soft” button, e.g., is a non-moving element that senses a touch and/or force exerted on a surface of the soft button.
A sensor, such as a capacitive fingerprint or touch sensor, may be positioned under the button. Generally, the black ink should to be thin enough to make the sensor sensitive, but also optically opaque to conceal the sensor and match the coloring of the opaque region.
A black ink having these properties may include a high percentage of carbon pigments, such as carbon black, to obtain adequate optical density. However, carbon pigments typically are conductive, which may affect the operation of a capacitive sensor located beneath the ink layer. Further, the relative thickness of the ink layer may increase the distance between the sensor and an object it attempts to sense, such as a finger atop the button. Generally, the sensitivity of a capacitive sensor varies inversely with the square of the distance between the sensor and sensed object, so relatively small changes in distance may have large effects on sensor performance. Additionally, particles, voids, and contamination in the black ink or paints may affect the performance of the sensor and cause functional errors in sensor readings. These issues increase as the thickness of the ink layer used to color the button increases. Therefore, a thinner, non-conductive (or less conductive) black ink may be useful.